Questing regarding lifting horizontal stabilizer.

kismet · #1 10-25-2018, 11:48 AM

Hi folks. First post from a new-to-me RV-6 owner. Purchased in May (made the front page when Doedo Schipper posted me picture) and have about 120 hours or so on "Kismet."

The other day I took a tow-pilot friend for a ride and in his exuberance to get going and help me move my plane from grass to pavement, he picked up my horizontal stabilizer to move my non-castering tail around. All before I could say anything.

In this gentleman's defense, he carefully placed his open palms under the inner-most and next to inner-most rivet lines corresponding to the nose ribs on the right stabilizer. I calculated that he had to lift about 75 lbs across his two hands in total. He said there was no creaking, oil canning, or anything untoward. He only did this because he was taught to move all tailwheel aircraft this way.

I've been losing some sleep over concerns of damage, despite having carefully inspected for any signs of tweaking or cracks by removing the rear fairing. All looks good. But the unsupported nose-ribs forward of the front HS spar give me pause and would certainly discourage me from ever lifting the plane the way he did.

I would never think to lift my tail this way, of course, having only resorted to lifting the tailwheel spring to manhandle the aircraft when left without a towbar. My napkin math after looking at drawings and my stabilizer suggest it ain't a good idea, but that the nose-ribs can support such a load without bending or fatiguing the attachment to the spar. The load is ideally partially shared by all rivets under compression on the top surface and tension on the bottom of the horizontal stabilizer skin, and the nose-rib is a solid plate that would be resistant to bending up or down.

Can someone reassure me that this load placed directly on the two inboard noseribs is probably fine "just this one time" so I can put it out of my mind.

It is for this very reason that I would never manhandle someone's plane (or even open their ****ing canopy) without asking them first.

JonJay · #2 10-25-2018, 12:37 PM

That area can be inspected through the lightening holes with a scope or even a light and inspection mirror if lightening holes where put in. That vintage, lightening holes may have been optional.
I would bet you are just fine, but a quick look for any deformation of the rib flange to spar, or any cracks in that area should be evident upon inspection.
If that is what you have already done, there isn’t anything else to help you feel better short of an engineering analysis. Sorry this happened.

kismet · #3 10-25-2018, 12:58 PM

Appreciate it. I'll stick a scope in the lighting holes to look at the rib not immediately visible when the fiberglass fairing is removed. I took a darn good look last night and didn't see anything and heard nothing when pushing up and down at various ribs lbs up and down. Not a creak.

rv8ch · #4 10-25-2018, 01:24 PM

I have not run the numbers, but I have built an HS for an -8. I would be really shocked if the HS is not strong enough to be lifted the way he did, or even from much closer to the tips. I'm sure others will chime in, but if you want the real final word, call the factory.

Raymo · #5 10-25-2018, 01:27 PM

Thinking this through a bit...

When flying, the HS is carrying some weight constantly. The airplane, when in aerobatic weight range, is capable of 6 Gs (6*75=450).

I know the 75 number is quite accurate since the wings are carrying most of the weight and CG also matters but even if you cut that number by 50%, it's a lot more than 75 lbs.

A thorough inspection certainly can't hurt, though.

EDIT: Snowflake is correct... the HS is pushing down during normal level flight, so the calculation should use the max negative Gs.

kismet · #6 10-25-2018, 01:59 PM

To clarify for everyone, we are talking about lifting under the ribs at the fuselage and one rib out from the fuselage. The idea being that the front edge is being lifted a few inches forward of the front spar. The perpetrator (passenger) in question had enough sense not to lift far outboard, but he probably didn't realize the leading edge is unsupported except for the ribs that are bolted two a spar near the middle and a spar near the elevator hinge.

Snowflake · #7 10-25-2018, 02:08 PM

Quote:

Originally Posted by Raymo

Thinking this through a bit...

When flying, the HS is carrying some weight constantly.

Think it through a bit more... The horizontal stab pulls *down* to counteract the pitching moment of the main wing. The action in question here was lifting *up* on the horizontal stab. As I recall, the ribs are symmetrical but the spar isn't.

Still, my gut feeling is that this wouldn't cause any damage as described. It's not like the person just randomly placed hands somewhere and hauled up, he knew enough to put the load on the structure directly and gave thought to where that structure was.

I wouldn't have been happy if a passenger just randomly did that either.

Aluminum · #8 10-25-2018, 03:12 PM

Quote:

Originally Posted by Snowflake

The horizontal stab pulls *down* to counteract the pitching moment of the main wing.

This is true but not pertinent to the calculation.

Keep in mind that your airplane has to survive > 100 mph gusts from any direction while tied down outside. The force and torque the stab attach is designed to withstand is far greater than just the weight of the loaded airplane while maneuvering.

If the culprit sat on the tip I'd be concerned. From the OP's description it sounds like a non-event.

Larco · #9 10-25-2018, 03:21 PM

Not saying that you should accept the lifting of your plane from the HS because it's your plane and your comfort level. But if you seen how many people have done it over the years and many times over the years you might relax more. It seems much easier on the back and body to lift from the HS then from the TW spring. Just a thought

David Paule · #10 10-25-2018, 05:23 PM

You'd have yield damage before any failure. That is, there would be some deformation that stays in place after the load is removed. If there's not any in the area loaded or in the structure between there and the mounting, you're okay.

The exception is that if something buckled it might unbuckle when the load is gone. But if that's the case, there would most likely be some sign of the buckling, some local yielding at the corners or something. If not, and if you didn't hear any oil-canning sound during the event, it should be okay.

After inspecting for that, keep an eye on the rivets in that area. Is there any sign that they've shifted in their holes? If not, and if they don't weep at condition inspections, they're okay too.

David Paule
Boulder
RV-3B building